In the United States, more than 541,000 individuals live with congenital upper-limb reductions or amputations. Worldwide estimates for upper-limb congenital reductions range from 4-5/10,000 to 1/100 live births. The use of body-powered upper-limb prostheses helps children with upper-limb reductions to engage in functional activities that are fundamental to normal growth and motor development. However, the development of prostheses for children is complex due to their rapid and continuous growth. Up to 58% of children with upper-limb reductions reject or abandon their prosthesis due to excessive weight, lack of visual appeal, limited function and complexity of control. 3D printed prostheses provide a cost-effective solution to the development of light-weight, customized and visually appealing prostheses for children, potentially encouraging use. Theoretically, the use of a prosthesis may lead to an enlargement of the primary neuronal networks located in the cortical area involved with motor control of the affected limb. Ultimately, this might lead to a larger repertoire of motor strategies and integration of the prosthesis into the motor control of the child facilitating prosthesis acceptance. However, there is little or no evidence supporting this hypothesis. The neural basis underlying motor performance in children using a prosthesis has been severely understudied resulting in minimal empirical evidence. This is largely due to i) the high prosthesis rejection rate and abandonment observed in this pediatric population making it difficult to properly monitor behavioral or neural changes before and after using a prosthesis, and ii) technological constraints of traditional neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and electroencephalography (EEG), in the assessment of brain function of pediatric populations. Functional near- infrared spectroscopy (fNIRS) has emerged as a practical neuroimaging technique that is less sensitive to noise and movement artifacts than EEG and fMRI, making it easier for children to tolerate testing. The use of fNIRS in conjunction with customized and visually appealing 3D printed prostheses would provide the unique opportunity to quantitatively assess the influence of upper-limb prostheses in the neural activation patterns of the primary motor cortex and motor performance of children. Our pilot work has shown a reduction of cortical activation, a more efficient motor response, and increased coordination after prolonged use of a 3D printed upper-limb prosthesis. This study will determine the influence of using a prosthesis on the neural activation patterns of the primary motor cortex in children with unilateral congenital partial hand reductions. The central hypothesis is that prolonged prosthesis use will result in a reduced primary cortex activation indicating that wearing a prosthesis may assist the primary motor cortex to produce a more refined, specialized, and efficient motor cortex response improving motor performance and the functional use of the prosthesis.